1. Field of the Invention
The present invention relates to flame resistant hydraulic fluids which are ecologically safe and have good lubricating properties, and to a process for energy transfer utilizing the same.
2. Description of the Background
Hydraulic fluids based in particular on mineral oils are in common use. However, in mining, for safety reasons, flame resistant hydraulic fluids are necessary and these are used in a large number of applications, such as in coal cutting machines, tunnelling machines, turbo couplings and hydraulic props. However, even outside the mining industry, flame resistant fluids are preferred wherever disastrous fires could be caused by the escape of combustible media from the closed hydraulic system.
The current flame resistant hydraulic fluids are in particular aqueous systems, either oil-in-water emulsions (HFA) or glycol-polyglycol-water mixtures (HFC). The chief disadvantage of the o/w emulsions is that they are metastable systems which can become critical in particular because of temperature variations and electrolytes (DE-A-3,508,946). Where thickened systems are concerned, such as are required to avoid leakage losses from the seals in pumps and valves and to form lubricating films between solid surfaces in frictional contact, the polymers used usually have little shear stability, if any.
HFC fluids based on monoglycols, oligoglycols and polyglycols have, in addition to substantial flame resistance, the advantage of being physiologically harmless and ecologically acceptable (P. Lehringer, Erdol und KohleErdgas-Petrochemie 41,230 (1988) ), which is particularly advantageous in mobile applications where leakages of hydraulic fluid often seep into the soil. These systems are also considered to be substantially shear-stable, which however can be viewed as a criticism since usually those polymers making the greatest contribution to the overall viscosity of the fluid are most susceptible to shear damage because of their chemical structure. A further disadvantage of the current HFC fluids is that the proportion of active ingredient must be very high so that a minimum viscosity is retained even at somewhat elevated temperatures. Water contents .gtoreq.50% are quite typical here (C. Rasp, Tribologie Schmierungstechn. 35, 185 (1988)). Moreover, the additive packages for producing good lubricating and anti-wear actions are very complex in these fluids. A need therefore continues to exist for hydraulic fluids of improved properties.